This disclosure relates generally to combustion systems for power plants, and more particularly to combustions systems having reduced nitrogen oxide emissions.
During a typical combustion process within a furnace or boiler, for example, a flow of combustion gas, or flue gas, is produced. Known combustion gases contain combustion products including, but not limited to, carbon, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur dioxide, chlorine, and/or mercury generated as a result of combusting fuels. Combustion gases also contain nitrogen oxides (NOX), usually in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Various technologies have been applied to combustion systems to minimize the emissions of NOX, however, further improvements are needed.
FIG. 1 shows a prior art combustion system 100. As shown, the prior art combustion system 100 includes a fuel lean main combustion zone 120, a reburn zone 124, and a burnout zone 126 stacked upwardly from the base of the prior art combustion system 100. These different zones of the prior art combustion system 100 are enclosed within a housing 110. Within the main combustion zone 120, the fuel undergoes combustion and forms a flue gas that flows upwardly to the reburn zone 124. As used herein, the term “flue gas” refers to the products of combustion, including but not limited to, carbon, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur dioxide, chlorine, NO, NO2, and/or mercury generated as a result of combusting fuels. Flyash is excluded from flue gas when computing residence times in the combustion system.
The amounts of fuel and air supplied to the main combustion zone 120 of the prior art combustion system 100 are selected to achieve fuel lean conditions therein. The term “fuel lean,” as used herein, refers to a condition having less than a stoichiometric amount of fuel available for reaction with the O2 in the air, i.e., a stoichiometric ratio (SR) of greater than about 1.0. The exact SR in the main combustion zone 120 of the prior art combustion system 100 varies depending on the fuel type and combustion system design, but generally ranges from about 1.05 to about 1.10. Flue gas produced in the main combustion zone 120 then flows to the reburn zone 124 and fuel is added to the flue gas through one or more reburn inlets 134. The amount of fuel added through the reburn inlets 134 is effective to produce fuel rich conditions in the reburn zone 124. The term “fuel rich,” as used herein, refers to a condition having more than a stoichiometric amount of fuel available for reaction with the O2 in the air, i.e., a SR of less than about 1.0. The exact SR in the reburn zone 124 of the prior art combustion system 100 varies depending on the fuel type and combustion system design but generally ranges from about 0.85 to about 0.95.